1. Field of the Invention
The present invention relates to apparatus for use in maintaining proper tracking of a record track on a record carrier and, more particularly, to such apparatus suitable for use with record carriers of different polarity.
2. Description Relative to the Prior Art
A typical video disc is 30 cm in diameter and has information recorded thereon in the form of optical structure (micron sized pits, bumps, apertures, etc.) along a generally circular or spiral record track. The width of the record track, and the spacing between adjacent record tracks, can be less than a micron. One difficulty encountered in the playback of such a record carrier is maintaining proper tracking of a read spot on the record track. Because of disc eccentricity, wobble, etc., during the recording process, the record track may not possess a perfectly circular or spiral configuration. Thus, the position of the read spot is constantly monitored and adjusted to maintain proper tracking of the read spot on the record track.
U.S. Pat. No. 3,876,842 discloses apparatus for reading a record carrier, such as a video disc, on which information is stored in the form of optical structure along a record track. Three spots of radiation are projected onto the record carrier. One of the spots is nominally positioned on the record carrier to interact with the record track and is used for playback of the recorded information. The other two spots "straddle" the record track and provide tracking error signals. Specifically, the tracking spots partially overlap opposite sides of the record track. During playback, the tracking spots are detected by respective photodetectors after interaction with the record carrier, thus producing a pair of tracking signals. According to that disclosure, a tracking error signal is derived by comparing the average amplitudes of the two tracking signals, perfect tracking being indicated by equal amplitudes. An off-track condition is indicated by one tracking signal having an average amplitude greater than the other, in which case a tracking error signal is produced which adjusts the scanning optics to restore proper tracking.
For reasons which are discussed in detail below, such a tracking system can be made to perform satisfactorily with either a "positive" polarity video disc or a "negative" polarity video disc, but cannot be used interchangeably with discs of different polarity. If all video discs were of the same polarity, there would be no problem. But because of the different approaches taken by video disc manufacturers, the different requirements of video disc recordings, and the lack of standardization in the video disc technology regarding disc polarity, video discs of both polarities are produced. A highly desirable improvement, therefore, would be a tracking system which can interchangeably accept video discs of opposite polarity, while still providing proper tracking.